Function: Establishes and maintains communications channels, so program on different devices can establish a link. Often combined with the Transport Layer and may be different for each Industrial Ethernet Protocol.

Simplex:Communications on the transmission channel allowed to occur in only one direction. Only one device is allowed to transmit; others only receive. The channel's full bandwidth is always available.

Full-duplex:Allows every device to both transmit and receive simultaneously. Every device (normally only two) are required to have two physical or logical transmission channels.

Half-duplex:Each device can both transmit and receive, but only one device can transmit at a time. The channel's full bandwidth is available to the transmitting device which can't receive while transmitting.